1. Field of the Invention
The present invention relates to a vacuum booster, i.e. vacuum servo unit for a vehicle braking system, and more particularly to a vacuum servo unit for reducing an operating input force to be applied when the unit is started to operate.
2. Description of the Prior Art
A conventional vacuum servo unit is disclosed in Japanese Utility Model Laid-open application No.2-99061, for example. The vacuum servo unit has a housing, which is separated by a movable wall member to define a constant pressure chamber and a variable pressure chamber. A cylindrical member extends from the central portion of the movable wall member out of the housing to receive an input member which includes a plunger and a rod. The plunger is slidably fitted into the cylindrical member in coaxial relationship therewith. The rod is connected to the plunger through a ball joint, and extends through the open end of the cylindrical member out of the cylindrical member. An air valve for controlling atmospheric air is formed on one end portion of the plunger at its side opposed to the open end of the cylindrical member. Also, a vacuum valve for controlling negative pressure is provided on the inner side of the cylindrical member, and placed at the outer side of the air valve so as to extend in the same direction as the air valve extends. Also provided is a control valve which has a cylindrical folding member, one end portion of which is provided with a seal portion opposed to the air valve and the vacuum valve, and the other end of which is air-tightly fixed to the inner side of the cylindrical member. A first compression coil spring is mounted between the seal portion of the control valve and the input member (or, the cylindrical member), to bias the seal portion toward the vacuum valve. A second compression coil spring is provided for biasing the input member against the cylindrical member to make the air valve contact the seal portion, and make the seal portion away from the vacuum valve, in its normal condition. A stopper is provided for limiting a displacement of the plunger biased by the second compression coil spring against the cylindrical member. A reaction mechanism is provided for applying a reaction force to the plunger in response to the advancing force applied to the movable wall member by the pressure difference between the constant pressure chamber and the variable pressure chamber. Then, the variable pressure chamber is adapted to communicate with the atmosphere through a clearance which is defined between the air valve and the seal portion, and the variable pressure chamber is adapted to communicate with the constant pressure chamber through a clearance which is defined between the vacuum valve and the seal portion.
In addition to the ordinary structure of the vacuum servo unit as described above, the unit disclosed in the publication has been proposed to function as follows. Namely, it is so arranged that an area of a pressure sensing surface, which biases the input member against the second compression coil spring when the seal portion of the control valve and the cylindrical folding portion are applied with a pressure difference between the negative pressure and the atmospheric pressure in the normal condition, is reduced by preventing a radially extending portion of the cylindrical folding portion at the other side thereof from communicating with the atmosphere. Whereby, reduced is an increase of operating resistance which is caused when a biasing force, which biases the input member in response to the pressure difference between the negative pressure and the atmospheric pressure when the unit is started to operate, is reduced by shifting a stage where the seal portion of the control valve contacts the vacuum valve, to another stage where the seal portion of the control valve comes to contact with the vacuum valve and where the air valve moves away from the seal portion of the control valve. Accordingly, the operating resistance force applied in accordance with a preload of the second compression coil spring and a preload of the first compression coil spring (the preload of the second compression coil spring, in the case where the first compression coil spring is mounted between the seal portion and the cylindrical member) is reduced to cause the operating input force to be small when the unit is started to operate.
According to the prior vacuum servo unit as described above, however, in the case where the diameter of the pressure sensing surface is made smaller than the diameter of the vacuum valve, when the servo unit operates, the air is introduced from the clearance between the seal portion and the air valve to cause a pressure difference between the atmospheric pressure and the negative pressure, by which the seal portion is biased to move away from the vacuum valve, so that the desired valve operation can not be obtained with the seal portion placed away from the vacuum valve.
In order to ensure the desired valve operation between the seal portion and the vacuum valve, therefore, the diameter of the pressure sensing surface has to be made larger by a necessary amount than the diameter of the vacuum valve. The biasing force, which is applied to the input member and caused by the pressure difference between the negative pressure and the atmospheric pressure when the unit is started to operate, is much reduced in accordance with the shift from the stage where the seal portion of the control valve comes to contact with the vacuum valve to the stage where the seal portion of the control valve contacts the vacuum valve and where the air valve moves away from the seal portion of the control valve. Therefore, the above-described vacuum servo unit is not so effective to reduce the operating input force when the unit is started to operate.